I have been using my Toshiba Satellite A-300 laptop for the past 3 years. For the past last 2 months it turns off automatically after 30-45 mins. When it re-boots and when we select "Start with Normal Mode" it turns off after 15 mins. If, when it re-boots and I select "Start in safe mode with networking " it runs for hours without shutting down.
I tried re-installating Windows Vista from the Hard disk recovery partition, but still the same.
You should go to service center. Because without investigate directly, it would be difficult to find out problem. It may for cooling fan or you may have need RAM up-gradation or it may other hardware problem.